There has been a growing interest in developing electronic devices using amorphous metal oxides as the semiconductor component. These devices can offer advantages such as structural flexibility, potentially much lower manufacturing costs, and the possibility of low-temperature ambient manufacturing processes on large areas. For example, amorphous oxide semiconductors can be used to enable new devices such as electronic paper, flexible organic light-emitting diodes (OLEDs), ultra-high resolution displays, radio-frequency identification (RFID) technologies, and transparent displays and circuits.
One of the key benefits to using amorphous oxides is the potential to use both vapor-phase and solution-phase deposition techniques to deposit the semiconductor as well as other materials needed to fabricate the devices. Yet, to further realize the processing advantages of amorphous metal oxide semiconductors, all active components of the device should be mechanically flexible and preferably, most of the components of the device should be compatible with, if not processable by, solution-phase deposition fabrication.
For example, thin-film transistors (TFTs) based upon various solution-processed or vapor-deposited metal oxide semiconductors have been developed. However, a critical component in TFTs is the gate dielectric layer, which comprises an electrical insulator material and prevents leakage currents from flowing into the channel when a voltage is applied to the gate. In addition to exhibiting low-gate leakage properties, a good dielectric material also needs to be air and moisture-stable, and should be robust enough to withstand various conditions that are common in device fabrication processes, with properties that are tunable depending on the type of semiconductor employed in the TFT channel. Furthermore, to enable a robust fabrication process and stable device operation, optimization of the multilayer TFT structure by using appropriate material combinations is necessary. Thus, the substrate surface may have to be treated or coated to be compatible with the overlying layers fabricated on top of it. In addition, after the device is completed, a top layer may be needed to protect the TFT stack from the environment during operation.
Although some polymers have been employed as dielectrics for metal-oxide-semiconductor field-effect transistors (MOSFETs), several limitations of current-generation polymeric dielectric have yet to be overcome. First, the leakage current densities of conventional polymeric dielectric films are relatively high (usually >1×10−7 A/cm2 at 2 MV/cm, >>1×10−5 A/cm2 at 4 MV/cm) especially after thermal annealing at temperatures of about 250° C. or higher. Second, very few polymeric dielectric materials are sufficiently soluble to be solution-processed, especially via inexpensive printing techniques. Among those that are solution-processable, they often cannot survive the conditions used in subsequent processing steps, which significantly limits their application in device fabrication. For example, for TFT device fabrication, the deposition of overlying layers such as the semiconductor layer, the conductor layer, and other passive layers by solution-phase process may require solvents that compromise the integrity of the dielectric materials. Similarly, most known solution-processable dielectric materials cannot survive vapor-phase deposition methods (e.g., sputtering), which are commonly used to process metals and metal oxides. Third, currently available polymeric dielectric materials often fail to achieve high surface smoothness, which is a prerequisite for stable TFT performance and operation.
Accordingly, there is a desire in the art to identify appropriate organic materials and/or design and synthesize new organic materials that are compatible with diverse substrates, conductor, and/or semiconductor materials such that they could be employed in the whole TFT fabrication process to meet one or more device requirements including low current leakage densities, high thermal stability, resistance to harsh chemicals used in patterning steps, tuned surface energies, good adhesion, good solution-processability, and/or low permeation to water.